The present invention relates to polymer membranes for separation processes, and more particularly to polymer membranes for pressure-driven separation processes or the like which may be formed using high temperature polymers offering excellent physical and chemical durability.
Filtration and/or separation processes may be grouped into three separate classes or categories. These include concentration-driven separations, represented by processes such as dialysis, electromembrane separations, used to separate dissolved charged ions, and pressure-driven separations which include the more familiar processes of micro-filtration and ultra-filtration.
Pressure-driven processes achieve the separation of suspended or dissolved particles of different sizes as the result of the capability of the particles to penetrate through or be retained by semi-permeable porous membranes of varying permeabilities and porosities. The porosity of the membrane determines whether the separation is termed micro-filtration, ultra-filtration or hyper-filtration.
Polymer membranes are used on a large scale in many industrial processes. Applications for such membranes include the desalination of sea water, the cleaning of industrial effluents, the fractionation of macro-molecular solutions in the the food and drug industries, and the controlled release of drugs in medicine. Membrane separations are in many cases faster, more efficient, and thus more economical than conventional separation techniques.
Micro-filtration membranes are used for the filtration of particles in the 0.1-2 micron size range, whereas ultra-filtration membranes can trap particles in the 0.001-0.1 micron size range. Typical species separable by micro-filtration include pollen, blood cells, and bacteria, the latter in some cases having particle sizes down to about 0.2 microns. Ultra-filtration membranes can retain species such as DNA, virus particles, and Vitamin B12 (with a particle size of about 30 .ANG.).
Composite membranes are also known, a common example being asymmetric ultrafiltration membranes. These structures comprise a thin particle-selective skin layer or surface membrane of very fine porosity disposed for physical support on a substructure or backing plate of coarser porosity. Composite membranes of this type improve mass transport in processes such as ultra-filtration and reverse osmosis wherein very fine pore sizes must be provided.
Conventional methods for making microporous membranes include the sintered particle method wherein a fine powder of the selected membrane material is processed by sintering at temperatures just below the melting point of the powder. The sintered products are typically films or plates having thicknesses in the range of about 100-500 microns. Polymers such as polytetrafluoroethylene, ceramics, glasses, or even metals can be formed into microporous membranes by this technique.
The particle size of the powder is the main parameter determining the pore size of the final membrane in this method. A common characteristic of particle sintering, however, is that it generally yields structures of relatively low porosity, for example in the range of 10-40% by volume.
Several alternative methods for making microporous membranes from polymers have also been developed. One such approach, involves the stretching of a homogeneous polymer film to cause partial fracture of the film and the formation of a fine pore structure therein. Another technique involves irradiating a polymer membrane with charged atomic particles in a particle accelerator or reactor, and thereafter etching the irradiated membrane to enlarge the particle tracks therein to pores.
Still another process for microporous membrane manufacture is the so-called phase inversion process. In this process, an insoluble or immiscible species such as water is introduced into a thin liquid film of a polymer dissolved in an organic liquid. Polymer films precipitated from solutions in this way develop a network of more or less uniform pores due to the presence of the immiscible phase during the precipitation process.
As the above description suggests, many of the commercially important methods for membrane fabrication require the use of polymer solutions, or polymers that can be etched or otherwise dissolved in controlled fashion. As a consequence, these methods are limited to polymers which have some solubility in organic solvents, a factor which significantly limits membrane durability.
It would be very useful if a method for manufacturing polymer membranes from more durable polymers could be developed, so that membranes exhibiting a higher level of chemical stability could be provided. Particularly useful would be a method for making such membranes from high strength, high temperature polymers such as polyether ketones, polyether ether ketones, and liquid crystal polymers.
Microporous polymer membranes having pore sizes in the 0.1-0.5 micron size range, if exhibiting porosities above 50% by volume, would obviously offer strong performance advantages over sintered polymer membranes of lower overall porosity. Applications for microporous membranes of this type, especially if offering the requisite high thermal and chemical stability, would include the sterilization of pharmaceutical drugs, the controlled release devices for drugs or herbicides, the removal of micro-organisms such as bacteria, yeast cells, or the like from aqueous solutions, and the filtration of organic or aqueous solutions of inorganic acids or bases which are often the byproducts of chemical processing in industries such as the electronics and chemical industries.